Recovery of silver from used photographic solutions

ABSTRACT

In an intermittently-operating electrolytic cell for the recovery of silver from used photographic bleach-fix solution, the cathode assembly and the solution are automatically separated when the plating current is switched off. A buffer tank upstream of the cell, from which solution is delivered to the cell by a metering pump during plating, has a bottom level switch that switches off the cell and feed and switches on an emptying pump for pumping the cell solution back to the buffer tank. The emptying pump is stopped by a cell bottom level switch; and a filling pump is started by a top level switch of the buffer tank, which also starts the plating cell and the metered feed, the filling pump being stopped by a top level switch of the cell. The cathode assembly is self-draining.

[451 Aug. 26, 1975 RECOVERY OF SILVER FROM USED PHOTOGRAPHIC SOLUTIONS James Sidney Bentley, London, England [75} Inventor:

[73] Assignee: Photographic Silver Recovery Limited, London, England [22] Filed: June 19, 1974 [211 Appl. No.: 480,828

Hcndrickson 204/109 Mahany n 204/109 Primary Examiner-R, L. Andrews Attorney, Agent, or Firm-Rose & Edell [57] ABSTRACT In an intermittently-operating electrolytic cell for the recovery of silver from used photographic bleach-fix solution, the cathode assembly and the solution are automatically separated when the plating current is switched off A buffer tank upstream of the cell, from which solution is delivered to the cell by a metering pump during plating, has a bottom level switch that switches off the cell and feed and switches on an emptying pump for pumping the cell solution back to the buffer tank. The emptying pump is stopped by a cell bottom level switch; and a filling pump is started by a top level switch of the buffer tank, which also starts the plating cell and the metered feed, the filling pump being stopped by a top level switch of the cell. The cathode assembly is selfdraining.

3 Claims, 3 Drawing Figures PATENTED 1 2 3975 SHEET 2 [If 2 RECOVERY OF SILVER FROM USED PHOTOGRAPHIC SOLUTIONS This invention relates to the recovery of silver from used photographic solutions and is an improvement in or modification of the invention described in US. Pat. No. 3,715,29l to James Sidney Bentley dated Feb. 6, 1973.

In that earlier patent there is recited the problem that arises if the silver concentration in a silver recovery machine operating by electrolytic plating of a cathode fluctuates during plating; and to solve that problem there is proposed a method of operating that keeps the silver concentration constant in a simple way. According to that method, an auxiliary tank is installed upstream of the silver recovery machine to receive the sil ver waste solution from the film processing equipment or other source, a pump operating at a constant flow rate delivers waste solution from this auxiliary tank to the silver recovery machine, the silver recovery machine is supplied with plating current set at an appropriately chosen constant level, and the pump and electrical current supply are operated intermittently in substantial time coincidence so that the pump is always feeding when the recovery machine is operating to plate silver on the cathode and whenever the pump is not operating the plating current is removed.

Although the outflow rate of waste silver solution from a film processor varies greatly, the silver concentration in the solution remains substantially constant. Therefore, the solution in the auxiliary tank has a substantially fixed silver concentration. The metering pump, when pumping, delivers this solution at a constant flow rate into the recovery unit and thus the flow conditions obtaining in the recovery unit, during the periods when it is actually in operation, are always the same in that a constant flow of solution enters with a constant silver concentration. This enables a fixed current amperage to be selected appropriate to this particular silver concentration and the problems associated with varying silver concentrations and a more or less arbitrarily selected current value disappear.

Only simple level switches are needed to effect the starting and stopping of the metering pump, correspondingly intermittent operation of the recovery unit being controlled by the same switch circuitry. The additional equipment necessary for implementing the invention is thus essentially simple and inexpensive in character and does not give rise to any special difficulties in installation or maintenance. Although the recovery unit may be running only intermittently it nevertheless can operate while running at maximum efficiency despite the fluctuations in outflow from the film processor.

A combined Bleach Fix Solution now extensively used to process colour print paper can be de-silvered and prepared for re-use by the system disclosed in said US. Pat. No. 3,715,29l.

However, the Ferric EDTA complex in the solution gives rise to the difficulty that it softens and re-dissolves the silver back from the cathode into the solution when the plating current is switched off. The plating bath may be fed, by the metered flow system described in our Patent, with a constant volume of silver in solution to match a preset plating current amperage it is then important to ensure that the silver already collected on the cathode is protected from the bleach effect of the solution during shut down periods of the apparatus.

In the early design of our equipment it was considered that, when the solution flow from the photographic processor to the recovery cell ceased during shut down, a low pre-set holding current should be applied to maintain the silver on the cathode in effect, counteracting the solvent action of the bleach. But experiments in this direction lead to problems of overplating the solution, causing the silver on the cathode to soften, blacken and re-dissolve back into the solution. It has further been discovered that too low a plating current allows the silver to re-dissolve, with a high loss of solid silver back into the solution.

It is therefore an object of the invention to eliminate the problem of silver loss back into the solution from the cathode during shut down periods.

According to the present invention, in the method of the above-referenced patent means are provided whereby the cathode and the plating solution are automatically separated from one another when the plating current is switched off. One way of accomplishing this is by an automatic drain and re-fill system for the plating cell which comes into operation as soon as the level of solution in the buffer tank reaches a low level switch. However, even if the plating cell is emptied, silver on the cathode will still be softened and dissolved by residual puddles of solution left on the cathode plates. Therefore, as a further feature of the invention, a selfdraining cathode assembly is employed. Conventionally, the cathode is of the multiple flat disc type, the discs being disposed horizontally on a vertical shaft driven by an electric motor above the plating tank. These flat cathode discs are now dished into a conical form so that the solution drains down to the centre. A series of holes drilled at the lowest level of each disc allows the surplus solution from each disc to drain off to the bottom of the tank leaving the cathode silver surface free of solution.

Alternatively, instead of the plating cell being drained and refilled, the self-draining assembly can be raised vertically, by electro-mechanical jacks or similar means, clear of the plating bath and lowered again each time the plating current is switched off and subsequently switched on.

One arrangement operating according to the invention is shown by way of example in the accompanying drawings, in which:

FIG. 1 is a general diagram of a silver recovery plant, and

FIGS. 2 and 3 are a diagrammatic cross-sectional elevation and plan of the cathode assembly of the recovery cell.

As in said US. Pat. No. 3,715,291, the silver recovery cell or plating tank 1 l equipped with an anode and a self-draining cathode assembly is preceded by a buffer or store tank 12, which receives the used photgraphic solution to be processed and from which the solution is pumped to the recovery cell intermittently by a metering pump 13, the flow rate of the pump being chosen to match the plating current employed as explained in our prior patent. The buffer tank 12 is provided with top and bottom level switches A and D; the recovery cell 11, similarly has two further level switches B and E. Apart from the metering pump 13, a cell emptying pump and delivery line 14 is provided for pumping solution from the recovery cell 11 back to the buffer tank 12, and a filling pump and delivery line for returning the solution from the buffer tank to the recovery cell.

Assume that the unit is on stand-by, with the recovery cell initially empty, the plating current switched off and all three pumps idle. The buffer tank 12 fills with used photographic solution until the top level switch A is reached. Operation of this switch starts the metering pump 13 and the filling pump 15 and switches on the plating current. The recovery cell 11 then fills rapidly until the level of the switch B at the top of this cell is reached, whereupon the filling pump 15 is switched off but the metering pump remains on. At this time the level in the buffer tank will have fallen to the store level indicated at C. With the metering pump operating, plating continues until the level in the buffer tank 12 has fallen to the bottom level switch D. The metering pump and the plating current are then switched off and the cell emptying pump 14 is switched on. The cell 11 rapidly empties until the bottom level switch E of the cell is reached, at which point the pump 14 is stopped and the cathode in the cell is dry. The level in the buffer tank 12 will then have risen to the standby level indicated at F and the unit remains on stand-by until the level switch A is again reached.

in order to avoid electrical continuity between the recovery cell solution and the solution in the buffer tank, air break arrangements are provided in the cell emptying and filling lines at 16 and 17.

FIGS. 2 and 3 show the self-draining cathode assembly. The cathode discs 18 are clamped at intervals up the cathode drive shaft 19 with intervening spacers 20. To promote draining, each cathode disc is of frustoconical form with drain holes 21 near the centre. Although as shown the cathode discs are assembled with the smaller diameters of their frusto-conical forms lowermost, they could be inverted, the solution then draining outward over the cathode discs instead of inward.

What I claim is:

l. A method for recovering silver from waste silver solutions, especially spent photographic processing liquids, said method utilizing: an electrically operated silver recovery machine of the type which includes an electrical supply for passing electric current between two electrodes, one of which is a self-draining cathode assembly, and through waste silver solution in said machine; a feed line via which waste silver solutions having predetermined average silver concentrations are supplied for processing; an auxiliary tank arranged to receive waste silver solutions from said feed line; and a pump arranged to deliver waste silver solutions from said auxiliary tank to said silver recovery machine; said method comprising the steps of:

sensing the level of waste silver solutions in said auxiliary tank;

operating said pump, in dependence on the level of waste silver solutions in said auxiliary tank, to deliver waste silver solutions at a constant flow rate from said auxiliary tank to said silver recovery machine, said pump being rendered inoperative when the level of waste silver solutions in said auxiliary tank recedes below a specified level;

in substantial time coincidence with operation of said pump, operating said electrical supply at a constant current insufficient to produce silver sulfide in waste silver solutions having said predetermined average silver concentration, said electrical supply being rendered inoperative when said pump is rendered inoperative whereby said recovery machine is operative substantially throughout times when said pump is operating but substantially not at other times; and

in substantial time coincidence with the rendering inoperative of said pump and said electrical supply, automatically separating the cathode assembly and the waste silver solution, whereby the cathode assembly is substantially only immersed in the solution when said recovery machine and said pump are operative.

2. A method according to claim 1, wherein in substantial time coincidence with the rendering inoperative of said pump and said electrical supply the solution is automatically pumped out of the silver recovery machine, said solution being pumped back into the machine when said pump and said electrical supply again become operative.

3. A method according to claim 1, wherein in substantial time coincidence with the rendering inoperative of said pump and said electrical supply the cathode assembly is automatically lifted clear of the solution in the silver recovery machine, said cathode assembly being re-immersed in said solution when said pump and said electrical supply again become operative.

IF l 

1. A METHOD FOR RECOVERING SILVER FROM WASTE SILVER SOLUTIONS, ESPECIALLY SPENT PHOTOGRAPHIC PROCESSING LIQUIDS, SAID METHOD UTILIZING: AN ELECTRICALLY OPERATED SILVER RECOVERY MACHINE OF THE TYPE WHICH INCLUDES AN ELECTRICAL SUPPLY FOR PASSING ELECTRIC CURRENT BETWEEN TWO ELECTRODES, ONE OF WHICH IS A SELF-DRAINING CATHODE ASSEMBLY, AND THROUGH WASTE SILVER SOLUTION IN SAID MACHINE, A FEED LINE VIA WHICH WASTE SILVER SOLUTIONS HAVING PREDETERMINED AVERAGE SILVER CONCENTRATIONS ARE SUPPLIED FOR PROCESSING, AN AUXILIARY TANK ARRANGED TO RECEIVE WASTE SILVER SOLUTIONS FROM SAID FEED LINE, AND A PUMP ARRANGED TO DELIVER WASTE SILVER SOLUTIONS FROM SAID AUXILIARY TANK TO SAID SILVER RECOVERY MACHINE, SAID METHOD COMPRISING THE STEPS OF: SENSING THE LEVEL OF WASTE SILVER SOLUTIONS IN SAID AUXILIARY TANK, OPENING SAID PUMP, IN DEPENDENCE ON THE LEVEL OF WASTE SILVER SOLUTIONS IN SAID AUXILIARY TANK, TO DELIVER WASTE SILVER SOLUTIONS AT A CONSTANT FLOW RATE FROM SAID AUXILIARY TANK TO SAID SILVER RECOVERY MACHINE, SAID PUMP BEING RENDERED INOPERATIVE WHEN THE LEVEL OF WASTE SILVER SOLUTIONS IN SAID AUXILIARY TANK RECEDES BELOW A SPECIFIED LEVEL, IN SUBSTANTIAL TIME COINCIDEDENCE WITH OPERATION OF SAID PUMP, OPERATING SAID ELECTRICAL SUPPLY AT A CONSTANT CURRENT INSUFFICIENT TO PRODUCE SILVER SULFIDE IN WASTE SILVER SOLUTIONS HAVING SAID PREDETERMINED AVERAGE SILVER CONCENTRATIONS, SAID ELECTRICAL SUPPLY BEING RENDERED INOPERATIVE
 2. A method according to claim 1, wherein in substantial time coincidence with the rendering inoperative of said pump and said electrical supply the solution is automatically pumped out of the silver recovery machine, said solution being pumped back into the machine when said pump and said electrical supply again become operative.
 3. A method according to claim 1, wherein in substantial time coincidence with the rendering inoperative of said pump and said electrical supply the cathode assembly is automatically lifted clear of the solution in the silver recovery machine, said cathode assembly being re-immersed in said solution when said pump and said electrical supply again become operative. 